Nushell

Nushell

The Paradigm Shift: Text Streams → Structured Data

Nushell fundamentally reimagines shell design. Traditional Unix shells pass unstructured text between commands, requiring grep/awk/sed to parse strings. Nushell passes structured, typed data—tables, records, lists—eliminating brittle text parsing.

Stop parsing strings, start solving problems.

# POSIX: Parse text, hope format doesn't change
ps aux | grep 'nu' | awk '{print $2, $11}'

# Nushell: Query structured data directly
ps | where name =~ 'nu' | select pid name

The same verbs (where, select, sort-by, group-by) work universally across all data sources—filesystem, processes, JSON APIs, CSV files. Learn once, apply everywhere.

Core Philosophy

• Structured data: Commands output tables/records/lists, not raw text
• Type safety: Data has types; errors caught early
• Immutability: Variables immutable by default (mut for mutable)
• Cross-platform: Internal commands work identically on Linux/macOS/Windows
• Separation of concerns: Commands produce data; pipeline transforms it; renderer displays it

Essential Syntax

# Variables (immutable by default)
let name = "value"
mut counter = 0
$counter += 1

# String interpolation
$"Hello ($name), count: ($counter)"

# Pipeline input variable
"text" | { $in | str upcase }    # $in holds pipeline input

# Conditionals
if $condition { ... } else { ... }

# Loops
for item in $list { ... }
while $condition { ... }

# Pattern matching
match $value {
    "a" => { ... }
    "b" | "c" => { ... }
    _ => { ... }
}

# Closures
{|x| $x * 2 }

# Custom commands with typed parameters
def greet [name: string, --loud (-l)] {
    if $loud { $name | str upcase } else { $name }
}

Pipeline Architecture: Input → Filter → Output

# Input (source): generates data
ls                              # Produces table of files

# Filter: transforms data
| where size > 1mb              # Keep rows matching condition
| select name size              # Choose columns
| sort-by size --reverse        # Order by column

# Output (sink): consumes data
| first 5                       # Take top 5
| save large-files.json         # Write to file (or display)

Data Types

Common Commands Quick Reference

Files & Navigation

ls **/*.rs                    # Recursive glob (no find needed!)
cd ~/projects
open file.json                # Auto-parses by extension
save output.json              # Saves structured data

Data Manipulation

where condition               # Filter rows
select col1 col2              # Choose columns
get column                    # Extract as list (not table)
sort-by column                # Sort
group-by column               # Aggregate
each {|x| transform $x}       # Transform items
update col { new_value }      # Modify column

Text Processing

lines                         # Split text into lines
split row ","                 # Split string
parse "{name}: {value}"       # Extract with pattern
from json / to json           # Format conversion

External Commands: Bridging Worlds

# External commands run automatically, output captured as text
git status

# Use ^ to force external version (bypass internal command)
^ls -la                       # System ls, not Nushell ls

# Capture stdout, stderr, and exit_code with complete
let result = (git push | complete)
if $result.exit_code != 0 {
    print $"Error: ($result.stderr)"
}

# Parse external output into structured data
docker ps --format json | from json

Bash to Nushell Translation

Error Handling

# Try/catch
try {
    risky-operation
} catch {|e|
    print $"Error: ($e.msg)"
}

# Optional chaining (safe navigation)
$record.field?                # Returns null if missing
$record | get -i field        # Same with command

Critical: Pipeline Input vs Parameters

Pipeline input ($in) is NOT interchangeable with function parameters:

# ✗ WRONG - treats $in as first parameter
def my-func [list: list, value: any] {
    $list | append $value
}

# ✓ CORRECT - declares pipeline signature
def my-func [value: any]: list -> list {
    $in | append $value
}

# Usage
[1 2 3] | my-func 4  # Works correctly

Why this matters:

• Pipeline input can be lazily evaluated (streaming)
• Parameters are eagerly evaluated (loaded into memory)
• Different calling conventions entirely

Type Signatures

def func [x: int] { ... }                    # params only
def func []: string -> int { ... }           # pipeline only
def func [x: int]: string -> int { ... }     # both

Row Conditions vs Closures

Many commands accept either a row condition or a closure:

# Row condition - auto expands $it
$table | where size > 100           # Expands to: $it.size > 100
$table | where name =~ "test"

# Closure - explicit parameter
$table | where {|row| $row.size > 100}
$list | where {$in > 10}

# Closures can be stored and reused
let big_files = {|row| $row.size > 1mb}
ls | where $big_files

Common Gotchas

1. Parentheses for subexpressions: (ls | length) not ls | length in expressions
2. Closures need parameter: each {|it| $it.name } not each { $it.name }
3. $in for pipeline input: "text" | { $in | str upcase }
4. External output is text: Pipe through lines or from json to structure
5. Semicolons separate statements on same line, not for line endings
6. ^ prefix: Force system command over Nushell builtin
7. each on records: Only runs once! Use items or transpose instead
8. Optional fields: Use $record.field? to avoid errors on missing fields

References

Read the relevant reference files based on the task at hand:

• commands.md - Command reference and POSIX translations
• data-types.md - Structured data handling (records, tables, lists, type conversions)
• pipelines.md - Advanced pipeline patterns, functional programming, streaming
• interop.md - External command integration, complete, extern
• scripting.md - Custom commands, modules, main, control flow
• production.md - Testing, validation, logging, error handling patterns

Plugin Development (for building Nushell plugins in Rust):

• plugins.md - Complete guide to building Nushell plugins
• Template: scripts/init_plugin.py and assets/plugin-template/

Running Nushell

nu                           # Interactive shell
nu script.nu                 # Run script
nu -c 'command'              # One-liner